Future-generation communication systems called 4th Generation (4G) are under active study to provide high-speed services with a variety of Quality of Service (QoS) requirements to users. Especially, research is being actively conducted on support of high-speed services by ensuring mobility and QoS to Broadband Wireless Access (BWA) communication systems such as Wireless Local Area Network (WLAN) and Wireless Metropolitan Area Network (WMAN).
In such a system, a transmitter (e.g., a base station or BS) can transmit to a receiver (e.g., a mobile station or MS) a signal weighted based on channel quality information by transmitter maximal ratio combining (Tx-MRC). While Tx-MRC is efficient in signal transmission between two points, namely point-to-point communications under a single-cell environment, it degrades overall system performance due to the influence of interference introduced from neighbor cells under a multi-cell environment.
To avert the problem, the communication system adopts beamforming to cancel interference. Beamforming is a signal transmission scheme using a beamforming weight vector. The BS transmits a signal an MS using a beamforming weight vector with which it receives a signal from the MS.
This signal transmission and reception scheme is based on the premise that downlink and uplink channels between the BS and the MS are symmetrical. The above beamforming scheme for interference cancellation is implemented using a smart antenna based on information, such as Degree Of Angle (DOA), on a deterministic channel model that models perfect spatial separation between a desired signal and a non-desired signal by beams.
As stated above, the beamforming scheme is realized on the deterministic channel model. In a Time Division Duplexing (TDD) communication system, for example, an uplink beamforming weight vector (i.e., an uplink beam pattern) is still valid for the downlink only if a desired signal and an interference signal are received and transmitted at different angles under a stable channel environment. However, the desired signal and the interference signal are not separated perfectly in space under a complex channel environment such as a scattering environment. In this case, a statistical channel model should substitute for the deterministic channel model.
For this purpose, the BS should represent the channels from an array antenna to a single antenna of the MS as a vector. The vector (i.e., channel vector), which usually has Gaussian characteristics, is not given as a function of one DOA. Under the scattering environment, the downlink differs from the uplink in link characteristics because the downlink and the uplink reflect noise statuses. Therefore, application of the uplink beamforming weight vector to the downlink degrades performance.
The beamforming scheme is based on the assumption that the BS calculates a beamforming weight vector using accurate channel information. The BS acquires the channel information by feedback resulting from downlink channel estimation of the MS. However, inaccurate channel estimation of the MS, a feedback delay of the MS, and a processing delay of the BS can cause channel estimation errors.
Sounding is another method for acquiring channel information in which the BS uses channel estimation information acquired from the uplink for the downlink. Similarly, the sounding-based channel estimation may have errors due to inaccurate channel estimation and different channel characteristics between the downlink and the uplink.
Accordingly, there exists a need for performing beamforming, taking into account channel estimation errors caused by different channel characteristics between the downlink and the uplink in a communication system.